There is current interest in capturing carbon dioxide (CO2) from industrial processes and sequestering (or storing) the captured CO2 in a way to prevent CO2 gas from entering the atmosphere. The product of combustion in the manufacture of power and in other combustion processes results in the emission of CO2 to the atmosphere. These CO2 emissions are believed by some scientists to contribute to global warming. As a result, CO2 is considered to be a Green House Gas (GHG).
Carbon dioxide sequestration is achieved by capturing the CO2, and storing it once captured, before it has a chance to enter the atmosphere. The U.S. Government may soon seek to minimize CO2 emissions by promulgating legislation to enact a “Cap-and-Trade” system, or by other means, such as an EPA edict. The European Union (EU) and other developed countries have already (or are about to) enact similar legislation to regulate the amount of GHG emissions.
The current methods available for capturing CO2 are varied. Regardless of the specific method used, the captured CO2 needs to be purified in order to meet the required standards for safe pipeline transmission and injection to the subsurface, wherein it can be sequestered (stored) for eternity. Until now, there has been no economic incentive to capture and sequester CO2, and, therefore, there has been little incentive to develop the technology necessary to carry out this sequestration step. With the impending legislation in the US and abroad, there will soon be an economic disruption to the status quo of simply discharging CO2 to the atmosphere.
For several years, there has been a debate on the impact of GHG on global warming, and at various times, individuals and companies have explored, through studies, the economic consequences of having to capture and sequester the CO2 released during the combustion process. The studies utilized existing technologies, and then applied an “add-on” technology to treat the captured CO2 to make it suitable for sequestration at supercritical pressure, such as, for example, to prepare it for subsurface injection in various suitable geological formations. The studies demonstrated that the consequence of CO2 sequestration have added a considerable economic penalty with regard to energy production costs in the form of additional capital expenditures and increased operating costs.
The United States Department of Energy (U.S. DOE) has been at the forefront of commissioning studies and has embarked on sponsoring several research and development (R&D) programs intended to look for the most economic means for producing power, while sequestering CO2. These programs are seeking new technology designed to have the lowest impact on cost of power to the U.S. industrial and residential consumer. In the studies focusing on various sequestration processes proposed to-date, the CO2 stream could be collected prior to venting, and next compressed in a multistage CO2 compressor to the specified super critical pressure. The compressed CO2 would then be sent via pipeline to the CO2 capture site for injection, typically under supercritical conditions, in the targeted geological formation.
For example, in power generation applications, recovery and capture of CO2 from these processes is desirable. As an example, the synthesis gas created in a high-pressure coal (or coke or biomass) gasifier comprises substantial amounts of carbon monoxide (CO). Conventionally, the synthesis gas is subjected to a number of steps, including gas cooling, gas scrubbing to remove chlorides, and reaction of the scrubbed gas and with steam in one or more CO-Shift reactors where the CO is converted into hydrogen and CO2 according to the following “CO-Shift Reaction” equilibrium reaction: CO+H2O═CO2+H2 (exothermic reaction).
Ideally, most of the CO can be converted to CO2 and captured, pre-combustion. The resultant synthesis gas stream, prior to capture, can contain approximately 50% CO2 (on a dry basis). Unfortunately, this stream typically also contains H2S and COS, both of which are undesirable constituents. Conventional removal technologies, such as RECTISOL and SELEXOL employ physical solvents such as methanol or dimethyl ether of polyethylene glycol (DEPG) to achieve the removal of H2S and CO2 through proprietary processes. Other proprietary processes, such as MORPHYSORB and PURISOL also employ physical solvents to remove H2S and capture CO2. Generally speaking, the above-mentioned processes each achieve the sequential removal of sulfur-containing constituents followed by the removal of the CO2 using a common solvent. The recovered stream containing the sulfur constituents is routed for processing (e.g., in a Claus plant), or a sulfuric acid manufacturing plant while the recovered CO2 stream, free from any sulfur-containing constituent, is vented to atmosphere.
There are differences in the current physical solvent processes that result in differences in both the capital and operating cost. However, each of these processes suffers from a common drawback: each process regenerates its solvent by releasing the entire amount of captured CO2 at relatively low pressures. This common problem results in the energy requirement to compress the entire captured CO2 from approximately atmospheric pressure to a super critical pressure needed for sequestration. There are variations in each of the process configurations that partially mitigate these problems by releasing some of the CO2 at modest pressure, but the majority of the CO2 is still released at close to atmospheric pressure. As a result, the overall cost of equipment and energy required for the CO2 compression (and subsequent purification) is a major cost burden on the current CO2 capture-compression processes.
Thus, a need exists for an alternative approach for capturing CO2 from a high-pressure gas stream. The approach should be applicable to a wide variety of processes and conditions, including, but not limited to, high-pressure synthesis gas and/or high-pressure natural gas originating from a variety of process or natural sources and locations. The approach should be both energy efficient and cost-effective, both in terms of capital and operating costs.